This invention pertains generally to the field of treating materials containing leadxe2x80x94such as waste materials and soilxe2x80x94with chemical compounds which serve to reduce the bioavailability of the lead. The materials containing lead include hazardous wastes. The present invention is particularly useful in the field of treating solid wastes containing unacceptable levels of leachable lead, in order to control leaching in the natural environment and during digestion after accidental ingestion.
Lead is one of the more common metals found in the environment, and also one of the more toxic. It is widely distributed in soil and at waste disposal sites due to its use in leaded gasoline, paint, batteries, and general widespread industrial use. The impact of lead on humans, particularly children, is being noted at increasing lower levels. Lead impacts both learning ability and behavior, particularly in children. For that reason, there is serious concern about levels of lead in the soil, and the impact of that lead on children. The Agency for Toxic Substances and Disease Registry has estimated that the number of children in the U.S. exposed to lead in soil or dust at levels of concern is between 5.9 and 11.7 million (Impact of Lead-Contaminated Soil on Public Health, ATSDR, 1992). Thus there is an increasing need to reduce the toxicity of lead in soil and waste, as well as to provide for safe disposal of lead-containing wastes.
Much of the focus of lead treatment studies until recently has been on the reduction of the leaching potential of lead from waste materials, particularly as measured in the U.S. Environmental Protection Agency""s Toxicity Characteristic Leaching Procedure (TCLP) test, which is used to classify wastes as hazardous. A number of treatment technologies have been developed for immobilizing lead in wastes using phosphate, so as to render the wastes non-hazardous. See e.g. U.S. Pat. Nos. 4,737,356, 5,037,479, 5,193,936, 5,245,114, 5,430,233, 5,512,702, 5,536,899, and 5,569,155.
Less work has been done on reducing lead availability to the body following (accidental) ingestion of soil or waste. Part of the reason for this lack of emphasis on human uptake following ingestion had been the lack of an effective means of monitoring uptake during the digestion process. However, recently developed laboratory tests provide relatively simple tests for simulating the uptake during digestion, notably the Physiologically Based Extraction Test (PBET) developed by Ruby et al. See Ruby et al., xe2x80x9cLead Bioavailability: Dissolution Kinetics under Simulated Gastric Conditionsxe2x80x9d, Environmental Science and Technology, 26:1242-1248 (1992); Ruby et al., xe2x80x9cIn Situ Formation of Lead Phosphates in Soils as a Method to Immobilize Leadxe2x80x9d, Environmental Science and Technology, 28:646-654 (1994); Ruby et al., xe2x80x9cEstimation of Lead and Arsenic Bioavailability Using a Physiologically Based Extraction Testxe2x80x9d, Environmental Science and Technology, 30:422-430 (1996), and the test developed by Gasser et al., xe2x80x9cLead Release from Smelter and Mine Waste Impacted Materials under Simulated Gastric Conditions and Relation to Speciationxe2x80x9d, Environmental Science and Technology, 30:761-769 (1996).
The Ruby et al (1992) Physiologically Based Extraction Test (or PBET test) involves a two step process. In the first step, the material being tested is exposed to an acidic (HCl) solution containing the digestive enzymes pepsin, citrate, malate, lactic acid, and acetic acid. The slurry is mixed for one hour at 37xc2x0 C., then a small sample is taken for dissolved metal analysis. The remaining solution is neutralized to pH 7 and bile salts and pancreatin are added. The slurry is mixed for 4 more hours, then the solution is analyzed for dissolved metals. The first step models the digestive conditions found in the stomach, while the second step represents the digestive conditions as the chyme (digesting material) moves out of the stomach and into the small intestines.
Many of the treatment processes for lead in industrial wastes or soil involve the addition of orthophosphate to form insoluble lead phosphates. See the patents listed above. Phosphate, and especially phosphate combined with chloride, can reduce the solubilized concentrations of lead from a contaminated soil in the simulated digestion test leachates, as shown in Table 1.
The dosages given in Table 1 above and in the Tables hereinbelow are based on the weight of additive to the weight of soil sample, both on an xe2x80x9cas isxe2x80x9d basis (i.e., wet weight).
In simulated digestion (PBET) tests run on contaminated soils, there is only a slight reduction in lead concentrations as the material goes from the acid to the neutral stages of the test. Since one would expect the solubility of lead to go down with the increase in pH from acid to neutral conditions, the results suggest that the lead is being complexed (and hence stabilized in solution) by the digestive enzymes. Lead concentrations in the leachate representing the intestinal phase of the simulated digestion tests are still well over the solubility levels found for the soil at neutral pH values. Further reduction in the lead concentrations requires either that the complexes between the lead and digestive enzymes be broken or that a more effective stabilization method be developed.
During digestion, the material passes first through the stomach, where it is subjected to quite acidic conditions, then into the small intestines where the pH is raised to near neutral. In both areas, digestive enzymes are introduced that aid in the breakdown of the food. The low pH of the stomach acid, combined with the complexing capacity of the digestive enzymes creates a very aggressive environment for lead solubilization from the ingested soil or waste. Most lead compounds are quite soluble in the acidic solutions found in the stomach. Once the lead is solubilized, the digestive enzymes can complex the lead to maintain it in solution as the stomach contents (or chyme) is neutralized. Further, the lead contents of most soils and many wastes is comparatively low (i.e., less than 1% by weight), which further enhances the solubilization of lead from the soil. Thus any treatment process that will immobilize lead during digestion must be particularly effective at lowering the solubility of lead, or in reducing the contact between lead and the digestive solutions.
As evidenced by Table 1, addition of phosphate or phosphate plus chloride can reduce the solubility of lead. Addition of phosphate results in the formation of the insoluble lead phosphate compounds, such as pyromorphite or chloropyromorphite. These compounds are the least soluble common lead compounds in environmental samples. The use of phosphate for immobilizing lead in regulatory leaching tests or in the environment has been documented. See Nagle et al., xe2x80x9cTreatment of Hazardous Foundry Melting Furnace Dusts and Sludgesxe2x80x9d, American Foundrymen""s Society Transactions, 87:767-785 (1983); Ruby et al., xe2x80x9cIn Situ Formation of Lead Phosphates in Soils as a Method to Immobilize Leadxe2x80x9d, Environmental Science and Technology, 28:646-654 (1994); Ma et al., xe2x80x9cIn Situ Lead Immobilization by Apatitexe2x80x9d, Environmental Science and Technology, 27:1803-1810 (1993); and Berti et al., xe2x80x9cIn-Place Inactivation of Pb in Pb-Contaminated Soilsxe2x80x9d, Environmental Science and Technology, 31:1359-1364 (1997). However, further reduction in lead solubility in the PBET test is needed to reduce the uptake of lead during the very acidic conditions encountered during digestion.
With no treatment, lead is very soluble in the acid pH region (pH  less than 5). In the stomachxe2x80x94with a pH of 1 to 3xe2x80x94lead in soil would be quite soluble, and hence available for uptake into the body. Addition of phosphate or phosphate plus chloride to lead-contaminated soil results in a much reduced solubility for lead in the acid pH region. The lead solubility is reduced through the formation of lead phosphates such as pyromorphite. See Nriagu, xe2x80x9cLead Orthophosphates-IV. Formation and Stability in the Environmentxe2x80x9d, Geochimica et Cosmochimica Acta, 38:887-898 (1974) and Ma et al., xe2x80x9cIn Situ Lead Immobilization by Apatitexe2x80x9d, Environmental Science and Technology, 27:1803-1810 (1993). The theoretical solubility of pyromorphite is quite close to that for the lead phosphate formed in the soil (Zhang et al., xe2x80x9cIn Vitro Soil Pb Solubility in the Presence of Hydroxyapatitexe2x80x9d, Environmental Science and Technology, 32:2763-2768 (1998). Conversion of the lead to a lead phosphate can reduce the solubility of lead under stomach acid conditions to some extent. However, the solubility of pyromorphite is still appreciable under acidic conditions. If the lead content of the soil is insufficient to reach saturation conditions, then the lead in the soil will still be appreciably soluble, even if in the pyromorphite form.
One method that can be used to decrease lead bioavialability is to introduce cations, such as calcium or magnesium, that compete with lead for digestive enzymes. This approach constitutes an aspect of the present invention.
Some treatment methods used to stabilize lead and render a waste or soil non-hazardous use calcium or magnesium compounds to accomplish this purpose. U.S. Pat. No. 4,889,640, for example, uses reactive calcium carbonate to render waste non-hazardous, while U.S. Pat. No. 5,037,479 uses magnesium oxide in combination with a phosphate compound such as calcium dihydrogen phosphate, commercially available as TSP (Triple Superphosphate). U.S. Pat. Nos. 5,193,936 and 5,569,155 teach the use of calcium sulfate (gypsum) in conjunction with phosphate to form materials termed superhard phosphates. U.S. Pat. No. 4,737,356 teaches the use of waste lime from a scrubber in combination with water soluble phosphates to stabilize lead in municipal incinerator ash. U.S. Pat. No. 5,569,152 teaches calcium oxide addition to electric arc furnace dust from steel manufacturing to form a solidified material which has low leaching potential in TCLP tests and low permeability.
In these processes, it is not calcium or magnesium which is the key treatment reagent. Rather, the calcium or magnesium is the cation associated with the desired anion, e.g. phosphate, sulfate, hydroxide, or oxide. The calcium or magnesium compounds may have the desired pH buffering properties for the process. A pH-neutral calcium or magnesium salt such as the nitrate could not be used effectively in these processes. In this invention, however, the anion associated with the calcium or magnesium is not crucial for the success of the treatment. Rather, it is presence of the soluble calcium or magnesium ions themselves that is important.
Many other treatment methods control lead leaching by pH control or by the formation of lead compounds that are more soluble than lead phosphate (e.g. lead carbonate). However these treatment methods will not be effective at reducing lead solubility under the very acidic conditions of the stomach, and thus will be ineffective at reducing the uptake of lead during digestion. Since one of the main routes of exposure for lead is through ingestion of contaminated material, and since lead poses a significant health threat to people, a method to reduce the bioavailability of lead form soil or waste during digestion is needed.
Another method to further reduce lead solubility would be to form a protective coating of an insoluble material around the soil particle that either buries lead under the coating or incorporates the lead as part of the coating. The coating material must be insoluble in the very acidic conditions found in the stomach.
One object of this invention is to develop a means of reducing the solubility of lead during digestion by forming a protective coating of an insoluble material around the soil particle that either buries lead under the coating or incorporates the lead as part of the coating. The coating material must be insoluble in the very acidic conditions found in the stomach. In particular, lead is incorporated into an iron oxide material in the soil that stabilizes the lead against solubilization even in the very acidic conditions of the stomach. The iron oxide material is formed through the in-situ oxidation of ferrous iron in contact with the waste material to be treated. Lead treated in this fashion is also stabilized against leaching in a TCLP test, and thus the treatment process is effective both for reducing the bioavailability of lead in soil or waste and for treating hazardous lead contaminated material.
Another object of this invention is reduction of the concentration and hence bioavailability of lead in contaminated soil or waste that may be accidentally ingested, by introducing cations that displace lead from the complexes present in the digesting material.
In one aspect of the present invention, lead can be stabilized in soil or waste by incorporating the metal into the solid formed by the oxidation of ferrous iron in contact with the soil or waste. Ferrous iron is introduced into the contaminated material, then oxidized. The resultant solid incorporates the lead and reduces its solubility, both in the natural environment and during digestion. This reduction of solubility can be verified by laboratory leaching tests such as the TCLP or PBET test. Acid generated during the precipitation of hydrous ferric oxide can be neutralized using common alkaline materials such as limestone, magnesium oxide, or magnesium hydroxide.
This invention accordingly provides a method comprising the steps of: (a) adding ferrous iron to the contaminated waste materials or soil, and (b) oxidizing the ferrous iron in the waste materials or soil by drying or by chemical oxidation to form a ferric compound incorporating the lead. In a preferred embodiment of this method, the ferrous iron is added to the contaminated waste materials or soil in an amount is calculated to provide a ferrous iron content therein within the range 0.25% through 5% by weight relative to the total weight of the treated waste materials or soil. The ferrous iron may be added to the contaminated waste materials or soil in the form of a liquid reagent containing a soluble ferrous salt, e.g., ferrous nitrate, ferrous sulfate, or ferrous chloride. Alternatively, the ferrous iron may be added to the contaminated waste materials or soil in the form of a dry reagent mix of a ferrous salt, and water may be added subsequently to dissolve the ferrous salt. In accordance with this invention, oxidation of the ferrous iron may be effected by atmospheric oxygen by aerating the soil or waste or by allowing the material to dry in the air, or, alternatively, by a chemical oxidant, e.g., hydrogen peroxide. This method can comprise a further step, step (c), of neutralizing the acid formed during the treatment process with an alkaline material, e.g., calcium carbonate, calcium hydroxide, calcium oxide, magnesium hydroxide, or magnesium oxide. Step (a) may, optionally, further include the addition of a mixture of phosphate and chloride to the ferrous iron and said waste or soil.
Another important embodiment of this method of the present invention contemplates the additional addition of a source of orthophosphate to the contaminated waste materials or soil. That orthophosphate source may be added in an amount calculated to provide an orthophosphate content in the contaminated waste materials or soil within the range 1% through 10%, preferably 0.1% through 5%, by weight relative to the total weight of the treated waste materials or soil. The source of orthophosphate may be, for instance, phosphoric acid, sodium phosphate, triple superphosphate, or combinations thereof.
Products produced by the above-described methods also constitute aspects of the present invention.
In another process aspect of the present invention, calcium and/or magnesium compounds are introduced into a soil or waste material contaminated with lead. As this material is (accidentally) ingested, for example as a contaminant of food, it passes through the stomach and intestines. Digestive enzymes in the stomach and/or intestines preferentially complex the calcium and magnesium rather than lead. At the neutral pH of the intestinal material, the lead then precipitates out and is unavailable for uptake by the body. This reduces the bioavailability of the lead. The calcium or magnesium can be added either as alkaline salts or as neutral salts.
Thus this invention also provides a method comprising the step of adding calcium or magnesium compounds to the waste materials or soil. The calcium or magnesium compounds, e.g., oxide/hydroxides or carbonates, or neutral salts such as chlorides or nitrates, may be added to the contaminated waste materials or soil in an amount calculated to provide a calcium or magnesium content therein within the range 0.5% through 10% by weight relative to the total weight of the treated waste materials or soil. This method may also include a step of adding phosphate or chloride or a combination thereof to the calcium or magnesium compounds and the waste materials or soil. The product of the method, namely, waste materials or soil comprising calcium or magnesium compounds and lead, also constitutes an aspect of the present invention.